Recent organizational changes, such as the increased emphasis on standardized procedures, in both the US and UK health care systems are likely to affect physicians' clinical decision making behavior (e.g., diagnoses, test-ordering, treatment). Older patients are more likely to experience the effects of these organizational changes due to the complexity and chronicity of the health problems that often accompany old age. This study will compare the clinical decision making patterns of physicians in the US health care system with their counterparts in the nationalized UK health care system, when presented with identical clinical information. This research by an experienced multidisciplinary international team builds on earlier successful work and proposes a factorial experiment to estimate simultaneously the relative influence of physician factors (clinical autonomy, gender, number of years since graduating medical school) and patient factors (age, gender, race and socioeconomic status) on the diagnosis and management of CHD and depression. Two hundred fifty-six physicians (128 in the US and 128 in the UK) will view two videotaped simulations of patients aged 55 and aged 75 consulting a primary care physician and presenting with either CHD or depression. The proposed experimental method, whose feasibility has been thoroughly demonstrated in prior studies, will identify and estimate both main and interactive effects among system, physician, and patient variables. Using 'think aloud' methodology to identify patterns of diagnostic considerations will allow estimation of physicians' cognitive processes underlying their decision-making in diagnosis and management of these two highly prevalent conditions. Generalizability of the findings is enhanced by focusing on two realistic clinical situations, and by randomly sampling and recruiting from physicians in primary care. The proposed study addresses a novel hypothesis - that international differences in disease largely result from variability in decision making (rather that geographic differences in disease associated with genetic background and life styles).